fix(cutoffs): use outward union for fitted arc bounds

docs/automatic-scrap-cutoffs.md

@@ -11,7 +11,7 @@ Finish nesting, then choose **Nest > Automatic Scrap Cutoffs (All Plates)** to p
 
 For the Nest command, the on-sheet overlay and upper measurements show the active plate; the scrollable diagnostics list reports every plate's proposed cuts, occupied/used spans, retained tail and warnings. Each plate uses its own dimensions, quadrant and part spacing with the dialog's common spacing/minimum-tail settings and current cutoff clearance/direction settings. No plate navigation or per-plate confirmation is required. Empty or unchanged plates are left untouched. A blocking conflict or invalid plate prevents changes to **all** plates, with the plate number reported; resolve it before retrying. If application fails after changes begin, all touched plates' original cutoff programs and sequence are restored. An explicit incomplete-recovery warning requires manual review before saving or cutting.
 
-An empty sheet is left untouched. Invalid input or out-of-sheet part geometry prevents automatic planning. Excessively small spacing is rejected rather than allocating an unbounded number of cuts. Ordinary floating-point roundoff between translated material and cached bounds is accepted without changing the part or its program; cutoff obstacle checks and fallback exclusions receive the same conservative numerical padding. Larger discrepancies (for example, an inconsistent arc center corrected during conversion) still prevent planning rather than risking a cut through material; repair the geometry before retrying.
+An empty sheet is left untouched. Invalid input or out-of-sheet part geometry prevents automatic planning. Excessively small spacing is rejected rather than allocating an unbounded number of cuts. Small bounded discrepancies between converted material and cached bounds—including amplified roundoff in fitted near-tangent arcs—are accepted without changing the part or its program. The same conservative numerical padding applies to cutoff pruning and exclusions. Their broad-phase envelope uses the farther edge of the cached box and converted perimeter; a cutoff within the narrow uncertainty band at a cached edge gets a full-envelope exclusion instead of relying on a near-tangent contour intersection. Elsewhere the contour is retained for precise trimming. Larger arc-center discrepancies still prevent automatic planning rather than risking a cut through material; repair the geometry before retrying.
 
 ## Preserving the unused tail